But with the price of uranium already soaring and plans to build over 120 new nuclear power plants around the world over the next 10 years, the economics and emissions benefits of nuclear energy may be in doubt, green groups said.

"Although we may not run out of uranium altogether, we could quickly run out high-grade, easily exploitable uranium," Roger Higman, campaign co-ordinator at Friends of the Earth, said.

He referred to studies showing that once high-grade uranium ore bodies had been exploited, lower-grade reserves would require a massive energy input to convert into fuel.

"That would affect the greenhouse impact of the nuclear sector and would make nuclear energy much more expensive."

Uranium prices have risen from US$8-10/lb four years ago to US$45.50 this week, according to US uranium consultancy UxC, and may head even higher.

"The key question is whether the high prices of uranium reflect a medium-term imbalance in supply and demand or whether this is a fundamental issue to do with dwindling reserves of uranium," Keith Allott, of environmental group WWF, said.

"High prices are likely to be a major factor in the economics of new nuclear plants. If the world embraces nuclear power as a so-called solution to climate change, we need to grapple with the fact that uranium is a finite resource."

INVESTMENT NEEDED Soaring oil prices and international attempts to reduce greenhouse gas emissions have thrown the spotlight back onto nuclear energy.

Proponents of nuclear energy say plenty of uranium is available, but the extended period of low prices has deterred investment in new mines.

"What is needed is new mine production, and at these prices there are huge profits available for miners so production will be strongly stimulated," Steve Kidd, director of strategy and research at the World Nuclear Association, said.

"The lead times for a uranium mine and nuclear reactor are about the same, so new mine capacity will come on stream at the same sort of time that the reactors come on."

He said the UK could see its first new reactors by 2015 at the earliest.

World uranium consumption is running at around 65,000 tonnes per year, while production is 41,000-42,000. The shortfall is made up from stockpiles held by utility companies and down-blended weapons-grade uranium.

A typical reactor consumes about 200 tonnes of uranium per year, but requires an initial charge of around 600-800 tonnes.

"Fuel prices don't affect nuclear energy economics. Fuel costs represent only a small component of the overall costs. Even if prices double it would remain a single digit percentage of the total cost," a source in the nuclear industry said.